1. Technical Field
The present invention relates generally to semiconductor devices and in particular to a process for manufacturing semiconductor devices. Still more particularly, the present invention relates to an improved method for forming tungsten plugs.
2. Description of the Related Art
The fabrication of semiconductor devices include processes that frequently require the deposition of thin layers onto the semiconductor surface. This deposition can be achieved by sputtering, but another important process for deposition of materials is chemical vapor deposition (CVD), which is regularly used for silicon dioxide, silicon nitride, and polycrystalline silicon (polysilicon). Basically, a material is reacted at a high temperature and is deposited on the surface of the semiconductor with the aid of a carrier gas. Vapor phase epitaxial techniques are a good example of this process. Deposition often takes place at low pressures. Improved film quality, however, can be obtained by allowing the reaction to take place at low pressures in what is known as a low pressure CVD (LPCVD) reactor. Chemical vapor deposition is not confined to polysilicon, silicon oxide, and silicon nitride. Other materials such as tungsten and tungsten silicide (WSi.sub.2) among others have been deposited using chemical vapor deposition.
Tungsten chemical vapor deposition (CVD) plugs are typically deposited by the reduction of WF.sub.6 by gaseous reducing agents such as silane (SiH.sub.4) and hydrogen gas (H.sub.2) are commonly used for vertically connecting adjacent layers of aluminum in sub 1.0 .mu.m applications. Tungsten is typically deposited on a titanium nitride (TiN)/titanium (Ti) liner in which the TiN layer acts as a diffusion barrier to prevent WF.sub.6 from reacting with the underlying layers. Titanium getters the interfacial impurities such as oxygen from the surface of the underlayers such as aluminum and silicon and reduces contact and via resistance. On other substrates such as silicon or silicon oxide, titanium also serves as an adhesion promotor for overlayers by forming a thin TiSi.sub.2 on silicon and TiO.sub.x Si.sub.y on silicon oxide layer.
Delamination of sputter-deposited TiN/Ti liners during the early stages of tungsten CVD is a major defect issue during semiconductor device processing. The delamination causes the formation of volcano-like defects that cause metal bridging or open metal lines, which often leads to device failure. WF.sub.6 diffuses through intercolumnar voids in the TiN layer and reacts with the underlying Ti layer during tungsten CVD. The resulting accumulation of large quantities of fluorine in the titanium underlayer interacts with the Ti/SiO.sub.2 interface, causing adhesion failure. Basically, fluorine diffuses through the bilayer of TiN and/Ti during the reduction of WF.sub.6 by SiH.sub.4 and .sub.2 H . The nucleation of tungsten using SiH.sub.4, and H.sub.2 or both is too slow to provide adequate protection from fluorine diffusion, depending on the structure of the TiN film.
Therefore, it would be advantageous to have an improved method for depositing tungsten on a semiconductor device.